The present invention relates to an electric power steering apparatus, and more specifically, to overheating prevention and temperature compensation in the electric power steering apparatus.
Generally, an electric power steering apparatus for applying a steering auxiliary force to a steering mechanism by driving an electric motor in response to a steering torque applied to a handle (steering wheel) by a driver has been used. The electric power steering apparatus is provided with a torque sensor for detecting a steering torque applied to the handle that is an operating member to steer, and a target value of a current to flow to the electric motor based on the steering torque detected by the torque sensor is set. Then, a command value to be given to a driver of the electric motor is generated based on the target value, and the voltage correspond to the command value is applied to the electric motor. The current is provided to the electric motor by applying the voltage.
In the above operation, a current flows into not only the electric motor but also the inside of an electronic control unit (ECU). As the current flows, heat is generated in the apparatus. As a result of the heat generation, the temperature of each of components constituting the apparatus gradually rises and the components may be damaged when the temperature of the components exceeds a predetermined temperature. In order to prevent the components from being damaged due to overheating, an upper limit of the value of a current for driving the motor has been set.
For example, in order to prevent the ECU from being overheated, the rise temperature of a component, such as a power transistor, etc., within the ECU is estimated and an estimated temperature of each component is calculated based on the estimated rise temperature of the component. Further, the estimated temperature of each component and the value of a current capable of being supplied to the motor may be caused to correspond to each other. The ECU is prevented from overheating by limiting the current value supplied to the motor based on the estimated temperature of each component (see, for example, Japanese Patent Publication No. 2003-284375A, and Japanese Utility Model Registration No. 2586020Y)
In a related technique, the weakest portion of an object to be protected from overheating is generally specified and protected. However, when there are various portions in which temperature rises to the highest depending on current output conditions based on steering situations, a plurality of objects should be protected from overheating. In this case, since it is required to provide a temperature sensor to each of the plurality of objects to be protected from overheating, cost increases. Further, since a temperature sensor is required to be provided in the vicinity of each object to be protected from overheating, a temperature sensor, the degree of freedom in designing a controller is lowered.
Further, a temperature sensor is required when compensating a temperature characteristic of a predetermined portion. For example, since the viscosity of grease for a reduction gear used to transmit a steering auxiliary force generated by the electric motor to a steering shaft has a temperature characteristic, a temperature sensor for detecting the temperature of a reduction gear is required for compensating the temperature characteristic.
Alternatively, overheating of the motor can also be prevented by estimating temperature rise of a mass part, a coil and a brush in the motor. In this case, the temperature rise of the mass part, the coil and the brush is estimated, and estimated temperatures of the respective portions are then calculated based on estimated rising temperatures of those portions. Further, the estimated temperatures of the respective portions correspond to a maximum value of a current which can be supplied to the motor. Also, the maximum value of a current which can be supplied to the motor is calculated by the estimated temperatures calculated for the respective portions by referring to the correspondence relationship. This limits the value of a current to be supplied to the motor and suppresses overheating of the motor.
Further, Japanese Patent Publication No. 2003-284375A discloses a motor temperature estimating device that determines whether an electric motor rotates or stops and then estimates the temperature of the motor in different calculating ways according to the determination results. The following points are described in this Publication. That is, the difference in temperature rise between a current flowing to a coil during the rotation of the electric motor and a current flowing to the coil during the stop of the electric motor is taken into consideration, so that the estimation precision of the temperature of the electric motor can be enhanced and the current can be controlled correctly.
However, in temperature estimating processing of the power steering apparatus in the related art, an error occurs between an estimated temperature and an actual temperature in devices such as a motor having, particularly, a large thermal resistance or a large capacity. These matters will be described referring to FIGS. 14 and 15. FIGS. 14 and 15 shows temperature change characteristics of a brush of a motor. FIG. 14 shows a temperature change characteristic of the brush in a case in which an input current is ON before a point of time denoted by a symbol t1 (a state in which a constant current flows) and the input current is OFF after the point of time denoted by the symbol t1 (a state in which the input current is cut off). FIG. 15 shows a temperature change of the brush in a case in which an input current is ON before the point of time denoted by the symbol t1 and after a point of time denoted by a symbol t2 and the input current is OFF during a period from the point of time denoted by the symbol t1 to the point of time denoted by the symbol t2. Here, when the temperature (actual temperature) of the brush falls down to the vicinity of the atmosphere temperature as shown in FIG. 14 after the input current is OFF, an error hardly occurs between an estimated temperature and the actual temperature even if the brush rises in temperature again. On the other hand, when the temperature of the brush does not fall down to the vicinity of the atmosphere temperature as shown in FIG. 15 until the input current is again ON after the input current is OFF, an error occurs between an estimated temperature and the actual temperature by the following reason. In this regard, in the temperature estimating processing in the related art, the estimated temperature is calculated according to a calculating formula that is defined based on actually measured data during temperature rise irrespective of whether the temperature is rising or falling. However, the temperature change characteristics of respective portions in the motor are different during temperature rise and during temperature fall. For this reason, in the temperature estimating processing in the related art, the estimated temperature when the temperature is falling cannot be obtained precisely, which causes the above-mentioned error. As a result, in controlling a current, the value of the current may be limited by a temperature different from the actual temperature, and the current may be excessively supplied to damage parts during overheating.